1. Field of the Invention
The present invention relates to a method for forming a PZT (lead zirconate titanate: Pb(Zr.sub.x Ti.sub.1 -x)O.sub.3) thin film, and more particularly, to a method for forming a PZT thin film using a seed layer.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a general PZT capacitor formed such that a PZT thin film is deposited on an electrode. The PZT thin film has good pyroelectricity, piezoelectricity and ferroelectricity and is widely employed for use in sensors, piezoelectric elements and memory devices. As shown in FIG. 1, a PZT thin film 14 is formed on a Pt electrode 13. In FIG. 1, reference numeral 11 denotes a Si substrate, reference numeral 12 denotes a SiO.sub.2 insulation layer, and reference numeral 15 is an upper Pt electrode. Here, in order for the PZT thin film 14 to exhibit a ferroelectric property, the PZT thin film 14 must have a phase having a special crystal structure, that is, a perovskite crystal structure. Lead (Pb) must be supplied sufficiently in an initial stage of seed formation in order to properly form a perovskite phase on a substrate made of a metal such as platinum (Pt). If Pb is not sufficiently supplied, it is difficult to form an initial thin film having a perfect perovskite phase, the initial thin film formed at an interface between a thin film and a substrate. Instead, the initial thin film may have both a pyrochlore phase and a perovskite phase.
FIG. 2 is a scanning electron microscopy (SEM) photograph illustrating the cross-section of the crystal structure of a PZT thin film 24 in the case when the PZT thin film 24 is formed on a Pt electrode 23 which is most widely utilized as an electrode. Here, the Pt electrode 23 is formed on a Si substrate 21 having a SiO.sub.2 insulation layer 22 coated thereon. As shown, since the PZT thin film 24 has different-sized grains, the surface of the PZT thin film 24 is rough due to protruding large grains. Such different grain sizes are attributed to different growth rates of the respective grains. In more detail, the grains grow at different growth rates according to their crystal orientation. It is known that the growth rate of PZT is highest in a &lt;100&gt; orientation, which is perpendicular to the surface of a substrate, and is lowest in a &lt;111&gt; orientation. Also, of the same orientation crystals, an earlier generated grain starts to grow earlier and thus is larger than a later generated grain, which is prominent in the case of a low nucleation density.
Also, in this case, the grain size is made larger than that in the case of a high nucleation density, so that the height of a protrusion becomes larger.
Also, the surface of the Pt electrode 23 is in good condition before pre-heating, as shown in FIG. 3A. However, the surface of the Pt electrode 23 becomes defective after pre-heating at 600.degree. C. for about 15 minutes, as shown in FIG. 3B, due to a difference between a deposition temperature and a pre-heating temperature. In other words, in the process of pre-heating, the Pt thin film experiences grain growth, thermal expansion/contraction, diffusion, oxidation, etc. It will also have larger grains, more intercrystalline defects, and a more rough surface all of which are impediments to the growth of the PZT thin film.
If the surface of the PZT thin film becomes rough by the above impediments, it is difficult to proceed with subsequent processes for device fabrication. Further, if an interface between the PZT thin film and an upper electrode becomes severely rough, the performance of a capacitor is degraded accordingly.
To date, in order to overcome such problems, there have been attempts to manufacture a seed layer by forming a uniform amorphous thin film at a low temperature and crystallizing the same through subsequent heat treatment, or to deposit a good thin film made of only a perovskite phase by facilitating generation of a perovskite seed by thinly depositing a material such as PbTiO.sub.3 as a seed layer and then depositing a PZT thin film thereon.
However, in the case when the amorphous phase is crystallized to be used as a seed, perfect crystallization of the amorphous phase cannot be achieved due to problems of composition change or recrystallization. Thus, since the amorphous phase remains between the substrate and the PZT thin film, the quality of the capacitor is deteriorated. Also, in the case of using PbTiO.sub.3, the generation of a seed is facilitated, compared to the case where PZT is directly deposited on a Pt substrate. However, since it is difficult to prepare high quality PbTiO.sub.3 and the surface of the thin film is rough, PbTiO.sub.3 is not suitable as a seed layer.